Question: Which of these numbers is prime? ${7,\ 9,\ 25,\ 49,\ 54}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 7 are 1 and 7. The factors of 9 are 1, 3, and 9. The factors of 25 are 1, 5, and 25. The factors of 49 are 1, 7, and 49. The factors of 54 are 1, 2, 3, 6, 9, 18, 27, and 54. Thus, 7 is a prime number.